It depends. Normally as soon as the threat is eliminated you're not allowed to use deadly force anymore, as far as I know. Since he took the knife from her, the knife wasn't a threat to him anymore, if he then cut her intentionally, he can be held responsible. If it's true what the article says and she continued to physically attack him while he had the knife and she got accidentally cut during the struggle then he can't be held responsible. At this point it's he-said, she-said, though, and now it depends on whom the judge will believe. Since she's already admitted that she attacked him with the knife, it's not looking good for her.